Magnetic recording media are widely used as audio tapes, video tapes or floppy disks. A magnetic recording medium fundamentally comprises a magnetic layer containing ferromagnetic particles dispersed in a binder which is laminated on a non-magnetic support.
High levels of various characteristics such as electromagnetic characteristics, running durability and running efficiencies are required for a magnetic recording medium. That is, higher abilities for reproducing original sounds are required for an audio tape used for recording and reproducing music. Also excellent electromagnetic characteristics, that is, excellent abilities for reproducing original images are required for a video tape.
Recently, short wave recording has been developed, and a magnetic recording medium using ferromagnetic alloy particles has been developed, as described in JP-A- No. 57-154163. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) It is known that the electromagnetic characteristics of a magnetic recording medium using ferromagnetic alloy particles fairly fluctuate depending upon the dispersibility of the ferromagnetic alloy particles in the magnetic layer. In other words, even though ferromagnetic alloy particles having excellent magnetic properties are used in order to improve electromagnetic characteristics, these excellent magnetic properties cannot lead to improved electromagnetic characteristics if the state of dispersion of the ferromagnetic alloy particles is poor.
Approaches for improving the state of dispersion of ferromagnetic alloy particles in a magnetic layer conventionally comprise mixing, kneading and dispersing the magnetic coating composition for a long period of time to prepare the magnetic coating composition for forming the magnetic layer, and adding dispersing agents. However, in these approaches, there are problems in that the magnetic properties decrease as a result of the mixing, kneading and dispersing for a long period of time and that the durability of the magnetic layer decreases by adding dispersing agents. It has been recently proposed that a polar group be introduced into the resins which form the binder so that the binder of the magnetic layer can have good affinity with ferromagnetic alloy particles.
For example, JP-A- No. 59-5424, discloses a magnetic recording medium which uses as a binder for the magnetic layer a resin having a predetermined polar group such as a metal sulfonate group in an amount of 50 wt % or more to improve electromagnetic characteristics, particularly at a short wave recording. As described above, ferromagnetic alloy particles are well dispersed in the magnetic layer by using a resin having a polar group as a binder for the magnetic layer. Thus, a magnetic recording medium having improved electromagnetic characteristics can be obtained.
On the other hand, fine particles of metal oxides and carbon black are included in a magnetic layer to maintain running durability which is evaluated in terms of head clogging of a video head or still life, as described in U.S. Pat. No. 4,613,545. Therefore, even though ferromagnetic alloy particles are well dispersed in the magnetic layer by using a binder having a polar group, sufficiently good electromagnetic characteristics cannot be obtained due to the undesirable effect of non-magnetic substances such as the fine particles of metal oxides or carbon black.